Electrolyte formulations for lithium ion batteries

ABSTRACT

Electrolyte solutions including combinations of high dielectric and low viscosity solvents. These solvent combinations provide low temperature performance and high temperature stability in lithium ion battery cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. non-provisional patentapplication Ser. No. 14/746,746 filed on Jun. 22, 2015 entitled“Electrolyte Formulations for Lithium Ion Batteries”.

BACKGROUND OF THE INVENTION

The present invention is in the field of battery technology and, moreparticularly, electrolyte formulations that enable both low temperatureand high temperature operation of lithium ion batteries.

Certain applications for lithium ion batteries require wide operatingtemperature ranges. In general, the power capability of lithium ionbatteries suffers at low temperature due to one or more of the followingfactors: 1) an increase in viscosity of the electrolyte resulting inslower lithium ion diffusion; 2) a decrease in the ionic conductivity ofthe electrolyte; 3) a decrease in ionic conductivity of the solidelectrolyte interphase (SEI) on the anode; and 4) a decrease in thediffusion rate of lithium ions through the electrode materials,especially the anode materials.

In the past, solutions to the problems associated with operating alithium ion battery at low temperature have involved adding solventsthat have very low melting points and/or low viscosity to theelectrolyte formulation. Such additional solvents can help prevent theelectrolyte solution from freezing or having substantially increasedviscosity at low temperatures. However, such additional solvents tend tobe detrimental to the high temperature performance of a lithium ionbattery, and in particular the high temperature cycle life.

Certain of the shortcomings of known electrolyte formulations areaddressed by embodiments of the invention disclosed herein by, forexample, improving power performance at low temperature withoutsubstantially decreasing high temperature cycle life.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention include a lithium ion battery cell having afirst electrode, a second electrode formed of lithium titanate and anelectrolyte solution. The electrolyte solution includes sulfolane and alow viscosity solvent, such as diethyl carbonate, methyl butyrate,methyl acetate, methyl propionate, isobutyl acetate, methyl trimethylacetate, methyl isovalerate, and combinations thereof. In someembodiments, the electrolyte solution does not contain ethylenecarbonate or propylene carbonate. In some embodiments, the electrolytesolution contains a blend of sulfolane and propylene carbonate.

Embodiments of the invention include a lithium ion battery cell having afirst electrode, a second electrode comprising lithium titanate, and anelectrolyte solution. The electrolyte solution includes a highdielectric solvent and a solvent such as diethyl carbonate, methylbutyrate, methyl acetate, methyl propionate, isobutyl acetate, methyltrimethyl acetate, methyl isovalerate, and combinations thereof. In someembodiments, the high dielectric solvent includes sulfolane. In someembodiments, the electrolyte solution does not contain ethylenecarbonate or propylene carbonate. In some embodiments, the electrolytesolution contains a blend of sulfolane and propylene carbonate.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1A and 1B illustrate electrochemical performance characterizationof battery cells assembled to contain electrolyte solutions made fromformulations of certain embodiments of the invention and controlelectrolyte solutions.

FIGS. 2A and 2B illustrate low temperature electrochemical performancecharacterization of battery cells assembled to contain electrolytesolutions made from formulations of certain embodiments of the inventionand control electrolyte solutions.

FIGS. 3A and 3B illustrate electrochemical performance characterizationbefore and after high temperature storage for battery cells assembled tocontain electrolyte solutions made from formulations of certainembodiments of the invention and control electrolyte solutions.

FIG. 4 illustrates low temperature and high temperature electrochemicalperformance characterization of battery cells assembled to containelectrolyte solutions made from formulations of certain embodiments ofthe invention and control electrolyte solutions.

FIG. 5 illustrates low temperature electrochemical performancecharacterization of battery cells assembled to contain electrolytesolutions made from formulations of certain embodiments of the inventionand control electrolyte solutions, where the solutions further includelow viscosity solvents.

FIGS. 6A and 6B illustrate low temperature and high temperatureelectrochemical performance characterization of battery cells assembledto contain electrolyte solutions made from formulations of certainembodiments of the invention and control electrolyte solutions.

FIGS. 7A and 7B illustrate low temperature and high temperatureelectrochemical performance characterization of battery cells assembledto contain electrolyte solutions made from formulations of certainembodiments of the invention and control electrolyte solutions.

FIGS. 8A and 8B illustrate low temperature and high temperatureelectrochemical performance characterization of battery cells assembledto contain electrolyte solutions made from formulations of certainembodiments of the invention and control electrolyte solutions.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions apply to some of the aspects described withrespect to some embodiments of the invention. These definitions maylikewise be expanded upon herein. Each term is further explained andexemplified throughout the description, figures, and examples. Anyinterpretation of the terms in this description should take into accountthe full description, figures, and examples presented herein.

The singular terms “a,” “an,” and “the” include the plural unless thecontext clearly dictates otherwise. Thus, for example, reference to anobject can include multiple objects unless the context clearly dictatesotherwise.

The terms “substantially” and “substantial” refer to a considerabledegree or extent. When used in conjunction with an event orcircumstance, the terms can refer to instances in which the event orcircumstance occurs precisely as well as instances in which the event orcircumstance occurs to a close approximation, such as accounting fortypical tolerance levels or variability of the embodiments describedherein.

The term “about” refers to the range of values approximately near thegiven value in order to account for typical tolerance levels,measurement precision, or other variability of the embodiments describedherein.

A rate “C” refers to either (depending on context) the discharge currentas a fraction or multiple relative to a “1 C” current value under whicha battery (in a substantially fully charged state) would substantiallyfully discharge in one hour, or the charge current as a fraction ormultiple relative to a “1 C” current value under which the battery (in asubstantially fully discharged state) would substantially fully chargein one hour.

To the extent certain battery characteristics can vary with temperature,such characteristics are specified at room temperature (about 25 degreesC.), unless the context clearly dictates otherwise.

Ranges presented herein are inclusive of their endpoints. Thus, forexample, the range 1 to 3 includes the values 1 and 3 as well asintermediate values.

The term “NMC” refers generally to cathode materials containingLiNi_(x)Mn_(y)Co_(z)O_(w), and includes, but is not limited to, cathodematerials containing LiNi_(0.33)Mn_(0.33)Co_(0.33)O₂.

Lithium ion batteries are assembled using an electrode pair (an anodeand a cathode) and a separator disposed between the anode and thecathode. Lithium ions are conducted by an electrolyte also containedwithin the battery cell. An electrolyte can include one or more solventsand one or more salts, such as a set of lithium containing salts in thecase of lithium ion batteries. Examples of conventional solvents includenonaqueous electrolyte solvents for use in lithium ion batteries,including carbonates, such as ethylene carbonate, dimethyl carbonate,ethyl methyl carbonate, propylene carbonate, methyl propyl carbonate,and diethyl carbonate.

Lithium titanate (e.g., Li₄Ti₅O₁₂; other stoichiometric ratios areincluded in the definition of lithium titanate) (“LTO”) can be used asan active electrode material for an electrode in battery cellapplications that require high power but do not require high energydensity. Batteries with LTO electrodes can operate at a potential of 1.5V, resulting in comparatively lower energy density relative to cellshaving a graphite electrode, which operate at a potential of about 0.05V.

In many lithium ion batteries using conventional electrolyteformulations, components within the electrolyte solution facilitate thein-situ formation of a protective film during the initial batterycycling. This is referred to as a solid electrolyte interphase (SEI)layer on or next to an anode. The anode SEI can inhibit furtherreductive decomposition of the electrolyte components. However, it hasbeen observed that SEI formation generally does not occur in batterycells with LTO anode operated at the comparably higher voltages asdescribed above. Recalling the factors above that are believed to limitlow temperature performance ((1) an increase in viscosity of theelectrolyte resulting in slower lithium ion diffusion; (2) a decrease inthe ionic conductivity of the electrolyte; (3) a decrease in ionicconductivity of the SEI on the anode; and 4) a decrease in the diffusionrate of lithium ions through the electrode materials, especially theanode materials), the lack of SEI on an LTO anode means that theelectrolyte formulation strongly influences the low temperatureperformance of batteries with LTO anodes.

At high temperature, stability of the battery cell can becomecompromised. Instability at high temperature is believed to be dueto: 1) increased reactivity of electrolyte with an active material; 2)accelerated decomposition of LiPF₆, which generates decompositionproducts that can be reactive with the both the electrolyte and theelectrode active materials. Parasitic reactions driven by thedecomposition products can result in loss of cell capacity and furtherdecomposition of any SEI.

Referring specifically to battery cells containing an LTO electrode, thehigh temperature stability of the electrolyte formulation can becompromised by catalytic effects of the titanium in certain oxidationstates. At a higher oxidation state, titanium tends to undergo a protonextraction reaction.

In view of the specific conditions present during the low temperatureoperation and high temperature storage of lithium ion battery cellsassembled to contain an LTO electrode, the electrolyte solution withinthe lithium ion battery cell should include solvents with good lowtemperature properties (e.g., low melting point, low viscosity, and highconductivity) and good stability in the presence of titanium ions (e.g.,Ti³⁺, Ti⁴⁺) at elevated temperatures.

Electrolyte formulations used in lithium ion batteries generally consistof at least two solvent types: a high dielectric constant (HD) solventand a low viscosity solvent (LV). The HD solvent is used to solvate thelithium ions that are conducted through the electrolyte. HD solventstend to have comparatively high viscosities and/or high melting points,which can be detrimental to low temperature performance. HD solvents asdescribed herein are solvents that have a dielectric constant of greaterthan about 40 at room temperature and are otherwise suitable for use ina lithium ion battery.

Thus, LV solvents are added to the electrolyte formulation to ensureadequate diffusion of the solvated lithium ions. However, LV solventsoften have comparatively lower thermal stability (they may have arelatively low boiling point, for example) and can compromise the hightemperature stability of the electrolyte formulation.

For reference, Table 1 presents melting points and viscosities forethylene carbonate, propylene carbonate, and sulfolane.

TABLE 1 Solvent properties Melting point Solvent (degrees C) Viscosity(cP) ethylene carbonate 36 1.86 (40 degrees C) propylene carbonate −49 2.5 (25 degrees C) sulfolane 27.5 10.3 (30 degrees C)

According to embodiments of the invention disclosed herein, the solventsulfolane (tetrahydrothiophene 1,1-dioxide; C₄H₈O₂S) is used in place ofthe conventional high dielectric solvents ethylene carbonate orpropylene carbonate. Structure (a) below represents sulfolane:

Most conventional electrolyte formulations contain ethylene carbonate(EC) as an HD solvent. EC plays an important role in the formation of astable SEI, especially with carbon-based electrodes. But, an SEI is notgenerally formed in the lithium ion battery cells using LTO electrodesunder the conditions described herein. Other carbonate solvents, such aspropylene carbonate (PC) are commonly used with carbon-based electrodesto partially replace EC to improve low temperature performance. As seenin Table 1, PC has a lower melting point and viscosity than EC. But, asdemonstrated by the results presented herein, PC is inferior tosulfolane in lithium ion batteries with LTO electrodes in spite of thehigher melting point and lower viscosity of the sulfolane.

Thus, an aspect of the inventive electrolyte formulations disclosedherein is the identification of the role of the inferior high dielectricsolvents EC and PC in the poor low temperature performance of lithiumion batteries with LTO electrodes. A further aspect of the inventiveelectrolyte formulations disclosed herein is the replacement of thosepoor performing solvents with the superior performing solvent sulfolane.

As described above, solvents that improve low temperature performancecan compromise high temperature stability. Further, combinations ofsolvents can behave unpredictably during charge and discharge cycles inelectrochemical cells. According to embodiments of the inventiondisclosed herein, certain low viscosity solvents can be combined withsulfolane in lithium ion batteries with LTO electrodes. Such lowviscosity solvents include certain ester-type solvents. Based on thetesting and analysis conducting herein, a set of exemplary solvents hasbeen identified to perform well in both the low and high temperaturetesting, and these solvents work well in combination with sulfolane inlithium ion batteries with LTO electrodes.

Low viscosity (LV) solvents as described herein are solvents that have aviscosity of less than about 1.0 cP at room temperature and areotherwise suitable for use in a lithium ion battery with respect toproperties such as electrochemical stability. Certain preferred lowviscosity solvents include methyl butyrate (C₅H₁₀O₂; structure (b));methyl acetate (C₃H₆O₂; structure (c)), methyl propionate (C₄H₈O₂;structure (d)), isobutyl acetate (C₆H₁₂O₂; structure (e)), methyltrimethyl acetate (C₆H₁₂O₂; structure (f)), and methyl isovalerate(C₆H₁₂O₂; structure (g)):

Other solvents may also be present in the electrolyte formulation. Forexample, linear carbonates, including but not limited to dimethylcarbonate, ethyl methyl carbonate, and diethyl carbonate, may be presentin the electrolyte formulation.

According to certain embodiments of the invention, sulfolane can be usedin solvent blends in combination with other HD solvents, such as PC.Further, these solvent blends can include a combination of linearcarbonates and one or more other LV solvents. Disclosed herein areseveral solvent blends that show unexpected performance at certaincritical volumetric ratios. The discovery of these solvent blendscapable of wide operating temperature performance relied on optimizingthe relationships between the HD and LV solvents. A series ofexperiments examining these relationships is presented below.

Low temperature performance is characterized by the area specificimpedance (ASI), which includes contributions due to the electrodematerials, the SEI layers formed on those materials, and the bulkelectrolyte properties. As this is a measure of impedance, low ASIvalues are desirable.

High temperature performance is characterized by measuring the change inASI after storage at elevated temperature. Again, small changes in theASI after storage are desirable, as that would indicate stability of thecell while stored at elevated temperature.

The following examples describe specific aspects of some embodiments ofthe invention to illustrate and provide a description for those ofordinary skill in the art. The examples should not be construed aslimiting the invention, as the examples merely provide specificmethodology useful in understanding and practicing some embodiments ofthe invention.

EXAMPLES

Battery Assembly. Battery cells were assembled in a high purity argonfilled glove box (M-Braun, oxygen and humidity content less than 0.1ppm). A LiNi_(x)Mn_(y)Co_(z)O₂ (x+y+z˜1) NMC material and a lithiumtitanate (LTO) material were used for the electrode pair (e.g., cathodeand anode). Each battery cell included a composite electrode film madefrom the NMC material, a polyolefin separator, and a composite electrodefilm made from the LTO material. Electrolyte formulations were madeaccording to the ratios and components described herein and added to thebattery cell.

Electrolyte Solution Formulation.

Electrolyte formulas included a lithium salt and a solvent blend. Thelithium salt was LiPF₆, and was used at a concentration of 1.2M. Thesolvent blends typically included a high dielectric (HD) solvent,certain conventional carbonate solvents such as ethyl methyl carbonate(EMC) and dimethyl carbonate (DMC), and optionally a low viscosity (LV)solvent.

Electrochemical Formation.

The formation cycle for these NMC//LTO battery cells was a 6 hour opencircuit voltage (OCV) hold followed by a charge to 2.8 V at rate C/10,with a constant voltage (CV) hold to C/20. The formation cycle wascompleted with a C/10 discharge to 1.5 V. All formation cycles were runat room temperature.

Electrochemical Characterization.

Initial area specific impedance (ASI) was measured after setting thetarget state of charge (SOC) (by discharging the cell at rate of C/10),and then applying a 10 second pulse at a rate of 5 C. Low temperatureASI results were derived as follows: The cell was recharged to 2.8 V ata rate of C/5 at room temperature, with a CV hold at C/10 followed by aone hour OCV hold. Then, the ambient temperature was reduced to −25degrees Celsius, followed by a 12 hour OCV hold to allow the test systemtemperature to equilibrate. All discharges to the specified SOC whereconducted at −25 degrees Celsius at a rate of C/10, with a one hour restat the specified SOC. A discharge pulse at 50% SOC was done at a rate of2 C for 20 seconds, followed by a 40 second rest. ASI was calculatedfrom the initial voltage (Vi) prior to the pulse and the final voltage(V_(f)) at the end of the pulse according to Formula (1), where A is thecathode area and i is the current:

$\begin{matrix}{{{ASI}\left( {\Omega \cdot {cm}^{2}} \right)} = \frac{\left( {V_{i} - V_{f}} \right) \times A}{i}} & (1)\end{matrix}$

After full recharge to 2.8 V at room temperature, the cells were thenstored at 60 degrees Celsius at OCV for two weeks. After two weeks thecells were removed from high temperature storage and then allowed toequilibrate to room temperature. The ASI was then measured by the sameprotocol used to determine initial ASI (setting the target SOC, and thenapplying a 10 second pulse at a rate of 5 C).

Results

FIGS. 1A and 1B illustrate electrochemical performance characterizationof battery cells assembled to contain electrolyte solutions made fromformulations of certain embodiments of the invention and controlelectrolyte solutions. In this testing, the solvent blend wasHD/EMC/DMC/MB at 20/30/40/10 by volume, where HD is sulfolane for thetest embodiment and HD is either EC or PC for the control embodiments.MB is methyl butyrate. The tested cells were charged to 2.8 V at a rateof 0.1 C with a CV hold to C/20 before discharge to 1.5V at 0.1 C.Referring to FIG. 1A, the first cycle discharge capacity (measured inmAh/cm²) was tested for a test embodiment and two controls. Referring toFIG. 1B, the coulombic efficiency (measured as a percentage) was testedfor a test embodiment and two controls.

FIGS. 1A and 1B demonstrate that no negative effect on initial dischargecapacity or coulombic efficiency was observed with the use ofelectrolyte solutions formulated according to certain embodimentsdisclosed herein. Thus, FIGS. 1A and 1B demonstrate that replacing EC orPC with sulfolane does not diminish certain electrochemical propertiesof the electrolyte formulation as compared to conventional formulations.

FIG. 2 illustrates low temperature electrochemical performancecharacterization of battery cells assembled to contain electrolytesolutions made from formulations of certain embodiments of the inventionand control electrolyte solutions. The cells were tested at a 50% stateof charge and at a low temperature of −25 degrees Celsius.

FIG. 2 demonstrates that electrolyte solutions formulated according tocertain embodiments disclosed herein showed superior low temperatureperformance as compared to control electrolyte formulations containingEC or PC. Superior low temperature performance in this instance ischaracterized by lower ASI at −25 degrees Celsius.

FIGS. 3A and 3B illustrate electrochemical performance characterizationbefore and after high temperature storage for battery cells assembled tocontain electrolyte solutions made from formulations of certainembodiments of the invention and control electrolyte solutions. As seenin FIG. 3A, the initial ASI before high temperature storage is similarfor the inventive and control electrolyte solutions. However, as shownin FIG. 3B, the final ASI (labeled as “2nd ASI”) after high temperaturestorage is lower for the sulfolane formulation than those containingeither EC or PC. FIG. 3B illustrates that there is a smaller change inASI during high temperature storage.

Thus, FIGS. 3A and 3B demonstrate that the use of electrolyte solutionsformulated according to certain embodiments disclosed herein showimproved high temperature stability as compared to control electrolyteformulations containing EC or PC.

FIG. 4 illustrates low temperature and high temperature electrochemicalperformance characterization of battery cells assembled to containelectrolyte solutions made from formulations of certain embodiments ofthe invention and control electrolyte solutions. The superior widetemperature range performance can be best observed by looking at bothaxes of the plot in FIG. 4, where the sulfolane formulation shows thebest performance at low temperature and after high temperature storage.The y-axis indicates the performance after high temperature storagewhile the x-axis indicates the performance at low temperature. Thus, theformulation with the best performance over a wide temperature range willhave values near the intersection of the two axes.

FIG. 4 demonstrates that the use of electrolyte solutions formulatedaccording to certain embodiments disclosed herein results incomparatively wider operating temperature performance relative tocontrol electrolyte formulations containing EC or PC.

FIG. 5 illustrates low temperature electrochemical performancecharacterization of battery cells assembled to contain electrolytesolutions made from formulations of certain embodiments of the inventionand control electrolyte solutions, where the solutions further includelow viscosity solvents. In this testing, the solvent blend wasHD/EMC/DMC/LV at 20/30/40/10 by volume, where HD is sulfolane for thetest embodiments and HD is PC for the control embodiment. LV wasselected from a variety of solvents, including diethyl carbonate (DEC),methyl butyrate (MB), methyl acetate (MA), methyl propionate (MP),isobutyl acetate (IA), methyl trimethyl acetate (MTMA), methylisovalerate (MI), and 1,2-dimethoxyethane (DME). The test conditions inrepresented in FIG. 5 were similar to those of FIG. 2.

FIG. 5 demonstrates that the use of electrolyte solutions formulatedaccording to certain embodiments disclosed herein show improved lowtemperature performance as compared to control electrolyte formulationscontaining PC. In this instance, a variety of low viscosity solventswere used in combination with the high dielectric solvents. Of the lowviscosity solvents tested, the class of solvents containing esterlinkages performed the best in this testing. And, the ester-containingsolvents performed best in formulations that also contained sulfolane.

The results disclosed herein demonstrate unexpected superior performancein certain solvent combination according to embodiments of the inventionherein. In a preferred embodiment, the high dielectric solvent sulfolanereplaces the high dielectric solvents ethylene carbonate and/orpropylene carbonate. As compared to prior systems of solventcombinations used in lithium ion batteries, the sulfolane combinationdisclosed herein does not include the high dielectric solvents ethylenecarbonate and/or propylene carbonate. This exclusion is preferred inlithium ion battery cells including an LTO electrode, as the LTOelectrode does not require the formation of an SEI under certainoperating conditions. Further, sulfolane can be combined with certainester-type low viscosity solvents to provide high temperature stabilityin addition to low temperature performance. These preferred solventcombinations yield superior performance in lithium ion battery cellsincluding an LTO electrode as compared to conventional solvent systems.

FIGS. 6A and 6B illustrate low temperature and high temperatureelectrochemical performance characterization of battery cells assembledto contain electrolyte solutions made from formulations of certainembodiments of the invention and control electrolyte solutions. In theseembodiments, the combination of HD solvents was tested with certain LVsolvents.

For these solvent blends, the solvents EMC and DMC were always present,while an additional LV solvent was varied. This collective component ofthe solvent blend is referred to as the non-polar (NP) component. Thatis, NP includes EMC+DMC+LV, where LV is chosen from MB, MA, and DEC.

The HD solvents were PC and sulfolane, and were tested in ratios of 1:3,1:1, and 3:1 by volume (indicated as 0.33, 1, and 3, respectively, alongthe x-axis of the FIGS. 6A and 6B). The NP solvents were tested in aratio of EMC:DMC:LV=3:4:1. And, the ratio of HD:NP=1:4. The controlformulation was PC/EMC/DMC/MB (2/3/4/1, v/v) 1.2M LiPF6 and isrepresented by a dotted line in the figures.

Therefore, the electrolyte formulations tested in FIGS. 6A and 6B can berepresented as:

1.2M LiPF6, PC/Sulfolane/EMC/DMC/LV (5/15/30/40/10)

1.2M LiPF6, PC/Sulfolane/EMC/DMC/LV (10/10/30/40/10)

1.2M LiPF6, PC/Sulfolane/EMC/DMC/LV (15/5/30/40/10)

Referring to FIG. 6A, which illustrates the results of low temperaturetesting, methyl acetate (MA), provided the best low temperature powerperformance for all of the HD ratios. With MA as the LV solvent, the lowtemperature power performance was directly related to the amount ofsulfolane in the blend.

Referring to FIG. 6B, the solvent blends showed similar high temperaturestability, with the most important factor being the ratio of PC tosulfolane. For high temperature stability, higher PC content waspreferred.

FIGS. 7A and 7B illustrate low temperature and high temperatureelectrochemical performance characterization of battery cells assembledto contain electrolyte solutions made from formulations of certainembodiments of the invention and control electrolyte solutions. In theseembodiments, the ratio of HD solvents to certain LV solvents was varied.

Again, the HD solvents were PC or sulfolane; the LV solvents were MB,MA, and DEC; and the NP solvents were EMC+DMC+LV (at a ratio of 3:4:1).The ratios of HD:NP were 1:2 or 1:3. The control formulation wasPC/EMC/DMC/MB (2/3/4/1, v/v) 1.2M LiPF6 and is represented by a dottedline in the figures.

FIG. 7A illustrates that sulfolane provided better low temperature powerperformance than PC, especially with MA and DEC. FIG. 7B illustratesthat sulfolane also provided superior high temperature stability ascompared to PC.

Preferred electrolyte formulations identified in this testing can berepresented as:

1.2M LiPF6, Sulfolane/EMC/DMC/MA (25/28.1/37.5/9.4)

1.2M LiPF6, Sulfolane/EMC/DMC/DEC (25/28.1/37.5/9.4)

Synergistic low temperature power performance was observed whencombining sulfolane with MA. The ratio of HD/NP solvent as well as HDand NP composition affected low temperature and high temperatureperformance, but overall the performance was dependent on the identityof the HD solvent. However, the optimal composition of PC or sulfolaneis not predictable based on their chemical properties.

FIGS. 8A and 8B illustrate low temperature and high temperatureelectrochemical performance characterization of battery cells assembledto contain electrolyte solutions made from formulations of certainembodiments of the invention and control electrolyte solutions. In theseembodiments, the ratio of HD solvents to certain LV solvents was variedalong with the ratios of sulfolane to PC.

The LV solvents were MB or DEC; and the NP solvents were EMC+DMC+LV (ata ratio of 3:4:1). The ratios of HD:NP were 1:2 or 1:3. The ration ofPC:sulfolane was 1:2, 1:1, 2:1. The control formulation wasPC/EMC/DMC/MB (2/3/4/1, v/v) 1.2M LiPF6 and is represented by a dottedline in the figures.

FIGS. 8A and 8B illustrate improved wide operating temperatureperformance for a variety of formulations.

An exemplary electrolyte formulations identified in this testing can berepresented as:

1.2M LiPF6, PC/SL/EMC/DMC/MB (12.5/12.5/28.1/37.5/9.4)

While the invention has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the invention asdefined by the appended claims. In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, or process to the objective, spirit and scope of the invention.All such modifications are intended to be within the scope of the claimsappended hereto. In particular, while the methods disclosed herein havebeen described with reference to particular operations performed in aparticular order, it will be understood that these operations may becombined, sub-divided, or re-ordered to form an equivalent methodwithout departing from the teachings of the invention. Accordingly,unless specifically indicated herein, the order and grouping of theoperations are not limitations of the invention.

1. A lithium ion battery cell, comprising: a first electrode; a secondelectrode comprising lithium titanate; and an electrolyte solution, theelectrolyte solution comprising sulfolane and diethyl carbonate.
 2. Thebattery cell of claim 1, wherein the electrolyte solution does notcontain ethylene carbonate or propylene carbonate.
 3. The battery cellof claim 1, wherein the electrolyte solution comprises propylenecarbonate. 4.-7. (canceled)
 8. A lithium ion battery cell, comprising: afirst electrode; a second electrode comprising lithium titanate; and anelectrolyte solution, the electrolyte solution comprising sulfolane andisobutyl acetate.
 9. (canceled)
 10. A lithium ion battery cell,comprising: a first electrode; a second electrode comprising lithiumtitanate; and an electrolyte solution, the electrolyte solutioncomprising sulfolane and methyl isovalerate. 11.-25. (canceled)
 26. Thebattery cell of claim 8, wherein the electrolyte solution does notcontain ethylene carbonate or propylene carbonate.
 27. The battery cellof claim 8, wherein the electrolyte solution comprises propylenecarbonate.
 28. The battery cell of claim 10, wherein the electrolytesolution does not contain ethylene carbonate or propylene carbonate. 29.The battery cell of claim 10, wherein the electrolyte solution comprisespropylene carbonate.
 30. The battery cell of claim 1 wherein theelectrolyte solution further comprises at least one linear carbonatesolvent.
 31. The battery cell of claim 30 wherein the linear carbonatesolvent is ethyl methyl carbonate, dimethyl carbonate, or a combinationthereof.
 32. The battery cell of claim 8 wherein the electrolytesolution further comprises at least one linear carbonate solvent. 33.The battery cell of claim 32 wherein the linear carbonate solvent isethyl methyl carbonate, dimethyl carbonate, or a combination thereof.34. The battery cell of claim 10 wherein the electrolyte solutionfurther comprises at least one linear carbonate solvent.
 35. The batterycell of claim 34 wherein the linear carbonate solvent is ethyl methylcarbonate, dimethyl carbonate, or a combination thereof.